Depends on the school. Some schools will send you two separate invites (one MD and one for MD/PhD). I deferred my MD interviews pending not getting a MD/PhD interview, because some schools will interview you for both at the same time depending on the MD/PhD interview decision. Sometimes the MD/PhD invite just comes a little later. Those schools were perfectly happy with that, and it seemed like a common situation. That being said, I then later declined the MD interviews at those schools that didn't give me MD/PhD interviews.
Which school are you referring to? If you don't feel comfortable saying you should just call them and ask. In fact, you should probably do that anyways.
